Good to hear from you Tony!
I'm leaning toward the 15400 series box. I'm not really keen on this extensive a re-wire, replacing the 'heart' of the car's system as it were. I've already had to remove & repair the circuit board on the '87, and expect that at some point it will be problematic to maintain. I figured when I have the entire dash out (some point in the future) to make the custom dash, it would be a practical time to do this.
In other news, I forgot to update on the targa/ sealing issues I had previously. I'm (cautiously) happy to report that the (now canvas) top no longer leaks at any of the points it did previously, it's also so much easier to R&R the top. The only things I'm trying to resolve are the bulb seal on the targa where the glass top edge meets, and a coating for the rubber locater blocks in the nose. The (Volvo) seal I'm using allows for a very slight gap, which creates a wind noise when driving with the top on, even if no water comes in. Looking for a seal with larger OD bulb (possible
example), hopefully offset slightly higher relative to the seam, as the installed seal is angled slightly upward relative to the horizontal plane. I don't want to adjust the door glass height any further as this then creates issues closing the door with the top on. The rubber blocks in the nose stained the canvas on the PS side - I was able to mostly clean it off, and have tissue tucked into them to keep the rubber from staining for now.